Thermoplastic pipe is produced from polymeric materials such as polyethylene, polypropylene, polyvinylchloride, polymethylmethacrylate, polyamide, polycarbonates, polyester and fluorinated polymers. Such pipe is used extensively in gas, sewer, water and drainage applications.
The extrusion of thermoplastic pipe is generally carried out horizontally, and there are two factors that contribute to circumferential nonuniformity of wall thickness of the pipe. The first is imperfect melt distribution or velocity of the polymer around the circumference of a given cylindrical melt path. In general, this imperfect melt distribution cannot be corrected by conventional die adjustments where the outer die is moved radially with respect to the mandrel.
A second factor causing nonuniform pipe wall thickness, especially on relatively thick wall products, is that gravitational forces acting on the still molten polymeric extrudate cause a downward flow of the molten polymer. For example, if the die orifice is adjusted so that the opening is perfectly concentric, the resultant pipe will generally be substantially thicker at the bottom portion than at the upper most portion even though the melt velocity at all points around the die orifice is substantially uniform. To compensate for this downward molten polymer flow, the outer die is generally moved upward with respect to the mandrel so that the die orifice is larger on the top than on the bottom to the degree necessary for the final solidified pipe wall thickness, as measured at the top and bottom, to be nearly equal.
Even with ideal die adjustment of a conventional pipe extrusion die as outlined above, significant pipe wall thickness nonuniformity still exists. In particular, significant increases in wall thickness often occur in the lower two quadrants of the cylindrical pipe; specifically 15.degree. to 60.degree. toward each side as measured from the pipe bottom point. No method currently exists to independently adjust the pipe wall thickness at these specific locations because the outer die has generally been moved with respect to the mandrel such that the solidified pipe wall thickness is nearly equal top and bottom, and nearly equal on each side, though the vertical average wall thickness need not necessarily equal the horizontal average wall thickness.
Aside from normal die adjustment where the outer die is moved with respect to the mandrel, additional localized adjustment of any thin wall cylindrical or conical section of the die would be difficult and impractical, since the inward deformation of such a section would result in uncontrolled bulging at another location about its circumference, and concurrent, uncontrolled redistribution of the polymer.